When manufacturing sulfate pulp for instance, lignocellulosic material, e.g. wood, finely divided to chips, and cooking liquor are charged to a digester. The cooking liquor may consist entirely of white liquor, i.e. fresh cooking liquor, or may consist of white liquor that has been mixed with a given quantity of cooking liquor that has already been used, i.e. waste cooking liquor. The chemicals present in the cooking liquor are caused to react with the wood chips at elevated temperature and elevated pressure. Subsequent to the cook, the resultant cellulose pulp is separated from the used cooking liquor, i.e. the waste cooking liquor, in diverse washing and/or pressing stages, these material flows being treated in the manner described below.
The cellulose pulp is normally screened and then subjected to at least one bleaching process as a rule. The pulp is normally bleached in several stages, often with intermediate alkali stages (extraction treatments). The pulp is normally washed after each bleaching stage, resulting in bleaching waste liquors. Waste liquor that derive from treatment stages in which the bleaching agents used contain no chlorine, such as oxygen, peroxide and ozone, are normally passed in counterflow to the pulp. treatment chain and finally mixed with the cooking waste liquor. The same applies to waste liquors that derive from alkali stages. When bleaching is effected with chlorine-containing bleaching agents, such as chlorine, hypochlorite and chlorine dioxide for instance, it is not normally possible to recover these bleaching waste liquors, but these liquors must be discharged to the recipient.
As above mentioned, the waste cooking liquor, also referred to as black liquor, may be mixed with recycled bleaching waste liquor. Irrespective of whether the black liquor consists exclusively of cooking waste liquor or of a mixture thereof, the black liquor is treated (for instance evaporation) so as to increase its solids content to 60-70% for instance. This liquor is then referred to as concentrated waste liquor. The concentrated waste liquor is burned in a recovery boiler in which the organic constituents of the concentrated waste liquor are converted essentially to carbon dioxide, water and energy, while the inorganic constituents of the concentrated waste liquor form a smelt in the bottom of the recovery boiler. The smelt is conducted along a drain or spout down into a soda dissolver, where the smelt is dissolved in weak liquor and/or water. The resultant solution is green liquor. The smelt is mainly comprised of sodium carbonate and sodium sulfide, which are both readily dissolved in water. The smelt also includes other elements, which originate mainly from the lignocellulosic material used as starting material in the digestion (cooking) process. Calcium and magnesium predominate among these other elements although iron, manganese, aluminium and silicon are also present in readily measured quantities. A number of other elements are also present in trace quantities (less than 1 mg per liter of green liquor). Distinct from the two sodium compounds, the aforesaid elements are insoluble or only partially soluble in green liquor. In sulfate pulp mills in which oxygen bleaching is employed and/or other bleaching processes with chemicals that do not contain chlorine are employed, meaning that all the bleaching waste liquor from this bleaching stage/these bleaching stages or a part of said waste liquor is passed back to the cooking waste liquor, primarily the magnesium content of the smelt increases. In turn, this is caused by the addition of some kind of magnesium compound as a protector in the oxygen and peroxide bleaching stages. Magnesium can also be added in pretreatment stages to the mentioned bleaching stages. The smelt also contains a certain amount of unburned carbon (soot).
The aforesaid insoluble or not readily soluble substances are present in the green liquor produced in the smelt dissolver in the form of solid particles. These particles must be removed from the green liquor to the greatest possible extent, and the removal of said particles in the form of sludge can be effected in several ways, as will be described in more detail further on.
The clarified green liquor is passed to a lime-slaker where quicklime, i.e. calcium oxide (CaO), is added to the green liquor. The calcium oxide is slaked in the green liquor and converted to calcium hydroxide (Ca(OH).sub.2), which in turn reacts with the sodium carbonate content of the green liquor, to provide a solution of sodium hydroxide and sodium sulfide and a calcium carbonate (CaCO.sub.3) precipitate, so-called lime mud. The lime mud is filtered off from the white liquor obtained by the described process, designated causticizing, and washed, preferably with hot water, to recover soluble compounds. The resultant weak liquor is normally passed to the aforementioned soda dissolver (smelt dissolver). The washed lime mud (CaC0.sub.3) is burned in a rotary furnace (lime kiln) to produce quicklime (CaO), which is returned to the lime-slaker to causticize the green liquor.
As earlier mentioned, the obtained white liquor is used as fresh cooking liquor for the digestion of newly charged lignocellulosic material.
If the earlier mentioned separation of sludge from the green liquor is insufficient, magnesium, aluminium, iron, manganese and other transition metals etc. will be concentrated in and contaminate the lime mud, which is formed in the lime-slaker and the causticizing vessels. The calcium carbonate content of the lime mud and also the content of free calcium oxide in the lime mud re-burned in the lime kiln will therewith fall progressively as the calcium oxide/lime mud circulates in the process, the so-called lime cycle. This makes it necessary to replace continually parts of the material in the lime cycle with pure limestone.
A typical method of removing the mentioned sludge is to sediment the sludge in a so-called green liquor clarifier. It is necessary to maintain the surface load on these clarifiers at a low level. A suitable green liquor input is 0.5 meters/hour. The cleaning result normally varies widely and the amount of sludge (contaminants) remaining in the liquor is rarely less than 50 mg per liter of cleansed (clarified) green liquor. When large volumes of green liquor are to be cleansed, about 4 m.sup.3 per tonne of pulp, very large clarifiers are required to achieve acceptable cleansing of the green liquor.
Attempts have been made to use conventional filters, such as pressure filters, vacuum filters, plate filters and drum filters instead of clarifiers, with the intention of partly decreasing the space requirements and investment costs and partly to achieve more effective cleansing of the Green liquor. However, it has been found difficult to filter green liquor instead of clarifying the same. The filters often become blocked and clogged too quickly, meaning that the various filters must be switched off and cleansed much too often.
The Swedish Patent Application 8700549-2 (456 254) describes a method which enables the aforesaid filters to be used to cleanse the Green liquor. According to this patent application, there is added to the unclarified green liquor while stirring a small amount of quicklime, namely 0.5-10%, preferably 1-3%, of the amount of quicklime required to fully causticize the green liquor, just prior to filtering (settling out) the Green liquor to remove the solid particles. According to the patent application, this procedure enables the green liquor to be filtered more effectively.
It is proposed in the Swedish Patent Application 9003697-1 (467 465) that a solution of slaked lime and green liquor in mixture is supplied to the unclarified green liquor instead of quicklime (in solid form) in precisely the same location. This liquid flow is comprised of part of the flow of slaked lime and green liquor that passes from the lime slaker to the causticizer(s) as a matter of routine. According to the patent application, this procedure will also enable the sedimentation and filtering properties of the green liquor to be improved.
Flocculating chemicals, an anionic polyacrylamide for instance, are also used to facilitate the separation of sludge from green liquor. Full scale tests carried out by us with this additive chemical show that the chemical can have a positive effect during periods in which the sludge can be separated relatively easily in the clarifier. This chemical provides no advantages during those periods in which sludge cannot be readily separated. When making a comparison between the occasion when a flocculating chemical is introduced and the occasion when no such chemical is introduced, no difference is noticed in the average value of the sludge content of the clarified green liquor over a long operational period, i.e. over a month or longer.